Particular patterning methods are used for producing extremely fine structures in the surface of a substrate such as, for example, microelectronic circuit structures or gratings for surface acoustic wave filters or semiconductor lasers. One of the most familiar methods known since the beginnings of semiconductor technology is optical microlithography. In this case, a radiation-sensitive photoresist layer is applied to the surface of a substrate that is to be patterned, and is exposed with the aid of electromagnetic radiation through an exposure mask. During the exposure operation, lithographic structures arranged on the exposure mask are imaged onto the photoresist layer with the aid of a lens system and transferred into the photoresist layer by means of a subsequent development process. The photoresist structures produced in this way are subsequently used as an etching mask during the formation of the structures in the surface of the substrate in one or more etching processes.
A principal objective of the semiconductor industry is continuous miniaturization of the structures, i.e. continuous reduction of the structure dimensions and of the distance between individual structure elements. In this case, the resolution limit that can be achieved for the structures is primarily limited by the wavelength of the radiation used. Therefore, ever shorter exposure wavelengths are also used in the course of the structure miniaturization sought. At the present time, the shortest exposure wavelengths used are 248 and 193 nm.
In order to increase the resolution limit in present-day optical lithography methods, special exposure masks such as alternating phase masks or binary masks with dipole illumination are used. In this way, a minimum center-to-center distance between the structure elements of 110 nm can be achieved by way of example in the case of a lithography process with an exposure wavelength of 193 nm.
In order to increase the resolution limit further, lithography methods which work with even shorter exposure wavelengths are being developed. These include lithography with an exposure wavelength of 157 nm and so-called EUV lithography with extreme ultraviolet light (EUV), in which the exposure wavelength is around 13 nm. A further lithography technique that is in development is so-called immersion lithography. In this case, the space between the lens system and the substrate to be patterned is filled with a liquid in order to improve the resolution capability. However, all these techniques will only be available in the future.
In order to produce very small structures, it is possible, furthermore, to use so-called electron beam lithography, in which a finely focused electron beam is directed over a photoresist layer that is sensitive to an electron beam. However, since each structure element has to be written individually in the case of this method, the method proves to be very time-intensive. Moreover, the method is relatively expensive on account of the low throughput resulting from this.
DE 42 35 702 A1 discloses a further method for producing very fine structures, in particular line or grating structures, in the surface of a substrate. This method is based on subdividing the structure to be produced into two partial structures, which are first of all formed as photoresist structures on the substrate surface in mutually separate lithography processes. The two photoresist structures are subsequently used as etching masks for producing the entire structure in the substrate surface in an etching process. Since adjacent structure elements of the structure produced in this way lie on a (center-to-center) distance grid that is half as large as that of structure elements in the individual partial structures, very fine structures can be produced by means of this method. In particular, it is possible to realize a distance between the structure elements which lies below the resolution limit of the underlying lithography processes. However, possible overlay errors of the two separate lithography processes prove to be a major disadvantage, as a result of which the imaging accuracy of the method is restricted.
A further method for producing very fine structures, in particular line structures, is disclosed in DE 42 36 609 A1. In this method, first of all an initial structure is produced on the surface of a substrate with the aid of a lithography process. Spacers are subsequently produced on the substrate surface, which laterally delimit the structure elements of the initial structure. After the removal of the initial structure, the spacers are used as an etching mask for producing a structure in the surface of the substrate. This method also makes it possible to realize minimum distances between structure elements of a structure below the resolution capability of the lithography process used for producing the initial structure. However, a major disadvantage of the method is that structure size fluctuations in the case of the initial structure bring about positional fluctuations of the spacers and thus of the structure elements of the structure to be produced. Consequently, the production of a regular structure in which all structure elements lie as far as possible exactly on a predetermined center-to-center distance grid is possible only to a limited extent.